As porous materials for use in the above-mentioned applications, there have been hitherto known those which are composed of organic polymers such as styrene-divinylbenzene copolymers, as well as those which are composed of inorganic materials such as silica gel. In use, they are generally packed in the form of a column.
The organic material columns have disadvantages such as that they have low mechanical strength against applied pressure, they easily swell or shrink when exposed to solvents, and they cannot be sterilized by heating. Thus, in cases where increased productivities are required by operation at high temperatures, the general practice is to use inorganic materials, particularly silica gel, because they do not suffer from such disadvantages.
Inorganic porous materials, such as silica gel are generally produced by the sol-gel process, which is a liquid phase reaction. As well known, the sol-gel process refers to a general process for producing aggregates or polymers of an oxide from an inorganic low molecular weight compound, in which the inorganic low molecular weight compound having hydrolysable functional groups for use as the starting materials is subjected to a sol-gel reaction, i.e., a hydrolysis and subsequently a polymerization reaction (polycondensation). The best known inorganic low molecular weight compounds for use as the starting materials are metal alkoxides. Other examples include metal chlorides, metal salts or coordination compounds having a hydrolysable functional group such as carboxyl group or β-diketone, and metal amines.
For use as a carrier for various purposes, a porous material should have an optimal median pore size as well as the narrowest pore size distribution suitable for the specific substance which is to be carried on the surface of the porous material for exhibiting the desired function. Thus, for a porous material produced by the sol-gel process, attempts have been made to control the pore size through controlling of the reaction conditions for the gel preparation.
Recently, a number of researchers have reported that the preparation of a porous material by the sol-gel process in the presence of an amphiphilic substance such as a surfactant or a block copolymer (more strictly, a molecular assembly of such amphiphilic substance formed through the self-organization) as a template makes it possible to precisely control pore structures in the nanometer range. However, conventional porous bodies produced by such sol-gel process generally have only nanometer-size pores (i.e., so-called mesopores), in which the bodies are mostly in the form of powder, thin films or irregular particles. Even if the porous bodies are produced in the form of a bulk material, no examples are found where larger-scale pores (i.e., so-called macropores) are controllably coexistent.
It is known that a sol-gel reaction for preparing silica gel using a basic catalyst, in the presence of an amide compound or from a silicon alkoxide as the starting material, will result in the enlargement of the average pore diameter. However, the resultant material has only pores of 20 nanometers, even at the largest, and a pore-size distribution in which most of the pores extend to the smaller diameter region.
In use as filters, carriers and other applications, the porous materials having only nanometer-sized pores (mesopores) as described above are generally pulverized and then packed in a column, in which the pulverized particles may be bonded together. Thus, the substance to be treated (i.e., the gas or liquid as the mobile phase) enters into the mesopores, through the spaces formed among the pulverized or bonded particles, in order to establish a desired function by the porous materials. However, many cases are known where the desired function is not fully exhibited because of insufficient porosity as well as the irregular or nonuniform spaces formed by the porous materials. A complicated and time-consuming process is required to produce a porous assembly by which a target substance can be smoothly introduced into nanometer-sized pores (mesopores) for contact therewith, or a bulk material having a macropores structure satisfying such condition.
It is an object of the present invention to provide a new method for producing inorganic porous bodies having precisely controlled macropores concurrently with mesopores of a narrow pore size distribution.